Dolphins sometimes have backward dorsal fins

*The following blog post is an excerpt from the book Twenty-Two Fantastical Facts about Dolphins*

Spinner dolphins are pretty bizarre as far as dolphins go. Their habit of leaping out of the water and spinning around at high velocity is just odd – not something other species of dolphins usually do. It’s made even more bizarre by the fact that nobody knows why they do it. It might be to shake off pesky suckerfish that are latched onto their sensitive skin. Or maybe it’s a form of communication, or a mating display. Or, as is likely the case for lots of dolphin behavior, it’s just something fun to do.

It’s not just that spinner dolphins act strange. They also look strange. An adult male spinner dolphin typically has a huge bump just behind its genital area called a ventral hump. The older a male dolphin is, the bigger its hump. So what’s in the hump? Nothing important really – mostly connective tissue. And what’s the point of a hump? Like many weird anatomical structures seen only on males of a species (like moose antlers or peacock tails), its sole purpose is to advertise how strong/awesome the male is, which both intimidates rivals and attracts the lady dolphins. Some other species of dolphins, like common dolphins, also grow a little hump for similar purposes. But nothing rivals the size and prominence of an adult male spinner dolphin’s crazy-looking hump.

Adult male spinner dolphins also have an impossibly strange-looking dorsal fin. It looks exactly like it’s been stuck on backwards. Instead of the fin curving from front to back – like you see in almost all other dolphin, shark, and fish species – it curves towards the front. As the males age, their dorsal fins change shape. When they are young calves, their fins are the normal size and shape. But once they reach full maturity, the fin loses its backwards curve and looks more like a triangle. As they continue to age, it starts pitching forward.

So what’s going on here? Probably more of the same “check me out ladies” kind of shenanigans that you find with their ventral humps. It might be the case that a backwards fin makes it harder to swim, so if an adult male can still manage to be at the top of his game with this kind of physical “handicap,” then other males know to leave this tough-guy alone. And the ladies will know that he’s probably good dad-material. Female spinner dolphins also have a reverse-fin sometimes, so it’s not just a male thing.

Male spinner dolphins also have odd-looking upturned or curled tail flukes. This might be yet another anatomical oddity that is meant to attract the ladies. Or it might help them swim faster. Nobody knows. Bottom line: adult male spinner dolphins look like they’ve been crafted out of playdough by a toddler who has no idea what a dolphin should look like.

Want to read more zany dolphin science trivia?  Order a copy of the book Twenty-Two Fantastical Facts about Dolphins